1. Field of the Invention
The present invention relates to an imaging device, such as a lens apparatus and a camera, which has a diaphragm control mechanism. More specifically, the present invention relates to a control apparatus and a control method capable of accurately controlling a diaphragm and capable of stabilizing a diaphragm action free from a hunting phenomenon.
2. Description of the Related Art
An imaging apparatus, such as a video camera, can include a position servo control mechanism capable of controlling a mechanical diaphragm provided therein. According to a conventional position servo control system, a Hall element can be used as a position sensor detecting the position of diaphragm blades (sectors).
The control system can obtain a difference between an output of the position sensor (the Hall element) and a target position of the diaphragm, and can produce a voltage signal representing the obtained difference. Furthermore, the control system can apply gain adjustment and phase compensation processing to the signal. The voltage signal is supplied to a diaphragm driving coil. The current corresponding to the applied voltage flows in the diaphragm driving coil. As a result, positional changes occur in the blades of the diaphragm and can be recognized as an output change in the position sensor. By repeating the above-described action, the control system can accomplish the diaphragm control.
However, when the position signal includes noises or any change occurs in frequency characteristics (for example, due to change of temperature), the diaphragm may not accurately stop at the target position and the diaphragm may cause a hunting phenomenon in the vicinity of the target position. That is, the diaphragm repeats opening and closing movements. If the servo action period is long, the hunting phenomenon appears as a change in the light quantity. If the servo action period is short, the diaphragm generates action sounds.
To solve this problem, it is possible to provide a neutral zone. According to a conventional technique, when a deviation between a detected diaphragm position and a target diaphragm position is less than a predetermined value, the deviation is regarded as 0 and no driving action is taken. For example, a neutral zone applicable to the exposure control of a camera is discussed in Japanese Patent Application Laid-open No. 10-276363.
Providing a neutral zone is generally effective to eliminate or suppress a hunting phenomenon. The stability of the diaphragm control can be improved. However, if an excessively large neutral zone is set, the deviation increases largely. On the other hand, if the deviation is decreased, the stability deteriorates.
FIG. 2A shows a relationship between an f-number indicating the state of the diaphragm and an output of the Hall element representing the diaphragm position. When the f-number increases discretely (i.e., by the steps of 1.4, 2.0, 2.8, 4.0 . . . ), the light quantity decreases in decrements of ½. However, the output of the Hall element (i.e., the diaphragm position) does not decrease in decrements of ½ and increases and decreases nonlinearly.
When the diaphragm value varies in the vicinity of a fully closed position of the diaphragm blades (refer to F22), a corresponding output change in the Hall element is small. On the other hand, when the diaphragm value varies in the vicinity of a fully opened position of the diaphragm blades (refer to the F1.4), a corresponding output change in the Hall element is large.
Accordingly, if a shifting motion of the diaphragm blades is controlled so as to produce a constant output change in the Hall element, the light quantity change is large in the f-number region corresponding to F22 and small in the f-number region corresponding to F1.4. As a result, as shown in FIG. 2B, the sensitivity in the light quantity change responsive to the position change of the diaphragm blades is high in the f-number region corresponding to F1.4 and low in the f-number region corresponding to F22.
Accordingly, when a neutral zone has a width corresponding to an output change of the Hall element responsive to the position change of the diaphragm blades in the f-number region corresponding to F1.4 (i.e., the region having a lower sensitivity in the light quantity change), the determined neutral zone is excessively wide for the f-number region corresponding to F22. Accordingly, the diaphragm stops with a large deviation in the light quantity when the f-number is equal to F22 or in its vicinity.
Therefore, in light of the aforementioned, it would be desirable to provide an apparatus and technique capable of setting an appropriate neutral zone for a diaphragm control apparatus and an optical apparatus including the diaphragm control apparatus.